A Pro-Russian activists holds a Russian flag during a rally in Donetsk on March 29, 2014. / Alexander Khudoteply, AFP/Getty Images

by Richard Wolf, USA TODAY

by Richard Wolf, USA TODAY

Diplomatic efforts to stabilize the situation in Ukraine moved so fast Saturday that Secretary of State John Kerry turned his U.S.-bound plane around and headed for peace talks with his Russian counterpart in Europe.

The situation on the ground remained less certain, however, as a top Ukrainian official warned of a potential Russian invasion - something Russian Foreign Minister Sergey Lavrov summarily disavowed.

En route home from Saudi Arabia, Kerry ordered his plane to do an about-face after a refueling stop in Shannon, Ireland, so that he could meet with Lavrov in Paris. State Department spokeswoman Jennifer Psaki told the Associated Press that the meeting would take place Sunday, probably in the evening.

Lavrov, meanwhile, gave an interview to state-controlled Russian TV on Saturday in which he said, "We have absolutely no intention of crossing Ukrainian borders."

"We simply want everybody to work together," Lavrov said. "We want the violence to stop, and we want the Western countries who are trying to sweep under the rug those cases of violence and to portray the situation in Ukraine in a positive light to realize they need to bear the responsibility."

At the same time, the foreign minister held out increased hope for a diplomatic solution.

"We're working on aligning our positions," he said. "Based on my latest meeting with Secretary of State John Kerry in The Hague and my contacts with Germany, France and a number of other countries, I can say that there's a possibility of drafting a joint initiative that we could offer to our Ukrainian colleagues."

That was of little solace to Ukrainian Prime Minister Arseniy Yatsenyuk, who told Fox News on Saturday that there remained "a huge possibility that Russia could invade and seize Ukrainian territory."

At the same time, Yatsenyuk told Fox that there was reason for optimism after the hour-long phone call between President Obama and Russian President Vladimir Putin on Friday. "Diplomacy is always the best way forward," he said.

The rapid-fire developments occurred as Obama completed a six-day whirlwind trip to Europe and Asia that included stops in Amsterdam, Brussels, Rome and Riyadh for NATO and European Union summitry, a visit with Pope Francis - and the unexpected phone call from Putin.

It was that phone call - reports of which varied, depending on whether U.S. or Russian officials were delivering the details - that set off the latest efforts at diplomacy.

U.S. officials said Obama proposed deploying international monitors in Crimea to protect the rights of Russian speakers, provided Russia halts what NATO has called a huge military buildup along Ukraine's eastern border and returns its troops in Crimea to their bases.

Russian officials said Putin had complained about activities of what he called "extremists" in Kiev, Ukraine's capital, and other regions.

Steven Pifer, a Russia expert at the Brookings Institute and a former U.S. ambassador to Ukraine, said Kerry's U-turn back to Europe was a good sign. But he expressed skepticism about Russia's moves and motives.

"It's a good thing that there's a conversation going," Pifer said. But he noted that "we continue to see the Russian massing of forces along the eastern Ukraine border. The question will be, does Lavrov have something new to say?"

What many foreign policy experts have called the gravest risk to East-West relations since the Cold War began just five weeks ago with the fall of Ukraine's pro-Russian president, Viktor Yanukovych, after months of protests.

Since then, Russia invaded and, after a hastily called referendum, annexed Crimea, an ethnic Russian region of Ukraine. U.S. and European nations, along with the United Nations and NATO, condemned the move and have imposed increasingly tough economic sanctions on Russian government leaders.